One of the most important known fault protection techniques for power lines (including power cables) is the so-called Overcurrent (OC) protection technique. See, e.g., GEC Alsthom Protection and Control Ltd, “Protective Relays Application Guide”, Third Edition, June 1987. Protection devices using the technique monitor the current in the power line through connections to current transformers in the line and when the current exceeds a predefined threshold, the protection device issues a trip signal to open a power line circuit breaker associated with the device. Such protection devices are conveniently referred to as ‘relays’ and they have current and time settings which are adjustable to grade the settings with respect to the settings of their neighboring relays to allow correct discrimination to be achieved during fault or overload conditions. In power networks having several sections of power line connected in series without significant impedance at their junctions, and where the source impedance is much greater than the impedance of the sections, there will be little difference between the magnitudes of currents which flow for faults in different positions on the network. In these circumstances, grading of the relays' current settings (“current grading”) is not able to offer satisfactory performance, so correct discrimination is obtained during fault conditions by using time graded relays, i.e., relays set to operate after different time delays. The timing difference between the relays associated with adjacent sections can be made sufficient to allow the appropriate circuit-breaker to open and clear the fault on its section before the relay associated with adjacent section nearer to the source can initiate the opening of its circuit breaker.
FIG. 1 shows a radially connected power line system provided with a known type of protection scheme based on time graded OC relays. Looked at from the source end of the system, a source S feeds power onto a busbar 1 and a line section L1 feeds current from busbar 1 to busbar 2. Current flowing in line section L1 near busbar 1 is measured by a current transformer CT, whose signal is passed to a relay R1 for controlling an associated circuit breaker B1, shown by the symbol X. The current fed into busbar 2 by section L1 is distributed to two further sections L21 and L22, which are connected to busbar 2 in parallel (though of course each of L21 and L22 considered individually is in series with L1). Sections L21 and L22 in turn carry the current to two further busbars 31 and 32, each of which have further line sections L31, L32, L33, etc., connected to them in parallel, and so on to complete the system. Hence, in such a power line system, current is said to radiate from the source end of the system to its far end through the above-described branching paths comprising “radially connected” power line sections. Similarly to section L1, each further line section L21, L22, etc., is provided with its own current transformer feeding line section current measurements to protection relays R21, R22, etc., with their associated circuit breakers B21, B22, etc. Each relay R1, R21, R22, etc., with its associated circuit breaker and line current transformer (which may be combined with a voltage transformer) is installed proximate, i.e., at or near, the electrically upstream end of each line section, this being the end nearest the source.
In such a power line system, with a single generating source at one end as shown, fault currents can flow in only one direction, i.e., from source to fault. To protect their line sections, each relay installed at a different position on the system is arranged to operate with a different time delay. The sequence from right to left is tf4→tf3→tf2→tf1, with operating times of 0.1→0.5→0.9→1.3 seconds, respectively. As can be seen from FIG. 1, the relays located in line sections nearer the source S have slower operating times than the relays in sections further away. A fault such as F1 in section L1 will cause higher current levels than a fault such as F3 in a section remote from the source S. In fact, fault currents in section L1 are likely to be so high that they can only be allowed to persist for a short period, which conflicts with the requirement for slower operating times. It will therefore be realized that although it is very easy to select the slower operating times needed to ensure that correct discrimination will be achieved when relays with predetermined time delays are used, their use must be restricted to networks with relatively few serially connected sections.